1. Field of the Invention
The present invention relates generally to personal communication devices. More specifically, personal communication method and apparatus, such as telephone handsets and headsets, with acoustic stray field cancellation are disclosed.
2. Description of Related Art
In personal communication devices such as telephone handsets and headsets, acoustic coupling between the receiver module (the speaker) and the transmit module (the microphone) results in some of the received signals appearing in the transmit path. Where the transmission delay (latency) is sufficiently long, such acoustic coupling between the speaker and the microphone causes the far-end talker to hear an annoying echo of his/her own voice. Thus, communication devices used in time-delayed networks, such as Voice over Internet Protocol (VoIP), should provide high levels of signal loss between the receive and transmit modules in order to minimize acoustic coupling.
In addition, speakers in telephone headsets or handsets should ideally produce low sound levels in the far field (stray field) in order so as to increase the level of privacy in the communication.
However, the electro-acoustic sensitivity of the receive and transmit modules of a headset typically must meet certain system requirements. In particular, the International Telecommunications Union, in combination with other international and national standards for telecommunications equipment, specifies values for electro-acoustic losses Relative Receive Loudness Rating (RRLR) and Relative Send Loudness Rating (RSLR), respectively, to ensure that when two people communicate via telephone (i.e., over a reduced frequency band), the acoustic loss from the mouth of the talker to the ear of the listener is the same as a face-to-face communication, as far as loudness is concerned. Loudness refers to the hearing sensation produced by an acoustic stimulus. Specifically, the RRLR and RSLR are specified to be 0 dB and 8 dB, respectively, in terms of loss relative to a specified Independent Reference System (IRS). The electro-acoustic loss RRLR represents the frequency-weighted average receive sensitivity of a telephone headset and is the ratio of the sound pressure at the user's ear drum reference point, DRP, to the voltage at the headset receive terminals. Similarly, the electro-acoustic loss RSLR represents the frequency-weighted average transmit sensitivity of a telephone headset and is the ratio of the voltage at the transmit terminals of the headset to the sound pressure at the user's mouth reference point, MRP.
Some personal communication devices have a relatively large distance between the microphone and the user's mouth and a reduced distance between the speaker and microphone. Examples of such devices include small cell phones and boomless headsets. Such a personal communication device should have a relatively more sensitive microphone and/or greater amplification in order to compensate for the larger distance between the microphone and the user's mouth. However, the reduced distance between the speaker and microphone results in increased acoustic coupling in the acoustic cross talk path. Thus, locating the microphone further away from the talker's mouth undesirably decreases the echo return loss at the telephone/network interface.
The echo return loss in a communication device is a function of frequency. A frequency-weighted average signal power loss between the electrical receive and transmit terminals of a communications headset is characterized by HCLw (Headset Coupling Loss, weighted). HCLw normalized with respect to RRLR and RSLR is referred to as Relative Terminal Coupling Loss, weighted (RTCLw).
Thus, it is desirable to achieve an ideal combination of receive sensitivity, transmit sensitivity, and receive-to-transmit coupling loss (HCLw) while at the same time maximizing RTCLw to provide full duplex telephone communication with high audio quality, particularly for boomless communication headsets and cell phones.
Conventional ear cups with an acoustic seal behind the speaker diaphragm and soft ear cushions on the face plate have been implemented to maximize the acoustic coupling to the user's ear and provide some of the desired properties such as reduced RRLR (loss) and increased HCLw. However, these headsets are relatively bulky and heavy and require headbands. In addition, although the use of noise canceling microphones especially with long microphone booms helps reduce RSLR (loss) and increase HCLw, the echo return loss performance achieved for headsets with short booms and boomless headsets are generally insufficient for digital networks.
Some conventional boomless headsets have decreased receive sensitivity and/or the transmit sensitivity below the recommended levels. As a result, these headsets do not provide satisfactory performance in noisy environments. One method to overcome this drawback is the use of form-fitting ear inserts on “ear bud” type headphones to create an acoustic seal between the receiver and the user's ear. Although such headphones increase acoustic isolation as well as receive sensitivity, such form-fitting ear inserts on ear bud type headphones are uncomfortable for some users.
Many ear bud and on-the-ear headsets and headphones have a rear opening or port to provide a vent for the backside of the speaker diaphragm. FIG. 1 is a cross-sectional view of an exemplary conventional headset receiver 20 shown in relation to a user's ear 40. The headset receiver 20 may be employed in an ear bud or on-the-ear headset or headphone. As shown, the headset receiver 20 includes an outer casing 22 defining a front port 24 and a rear port 26. The front port 24 is located on the headset receiver 20 such that when the receiver is placed in the user's ear 40, the front port 24 is positioned adjacent or otherwise near the ear canal 42 of the ear 40. The headset receiver 20 further includes a diaphragm 28 driven by a voice coil 30 and a magnet 32. The diaphragm 28, supported by a front plate 34, divides the volume defined by the outer casing 22 into a front cavity 44 and a rear cavity 46. The front plate 34 and a back plate 36 are used to complete the magnetic circuit and used to direct the magnetic field to a focal point in a gap formed by the front plate 34 and a pole piece 38.
The rear port 26 in the exemplary conventional headset receiver 20 is provided to increase the low frequency response of the receiver 20. The rear port 26 also provides an added side benefit in that the acoustic output of the rear port 26, which is out of phase with the front port 24, results in acoustic cancellation generally in the far field (stray field) and specifically at a transmit microphone, thereby improving the echo path loss. However, as is typical with conventional headsets, the acoustic cancellation achieved by the rear port 26 is effective only at low frequencies. The acoustic cancellation diminishes at mid-frequencies and becomes a detriment at high frequencies when the acoustic outputs of the front and rear ports 24, 26 cause constructive interference as will be further described in the detailed description of the invention. The upper curve of the graph of FIG. 2 illustrates the echo frequency response of an exemplary conventional boomless headset with a rear port. As is shown, although the echo level of a typical conventional headset with a rear port is low at low frequencies, the echo level rises steeply with increased frequency.
Two conventional solutions to attempt to resolve the problem of diminishing acoustic cancellation at higher frequencies are voice switching and voice expansion employing signal compression in the transmit channel. In both voice switching and voice expansion, the transmit gain is a function of the transmit signal level. In voice switching, the transmit gain is switched between a high state (when the user is talking) and a low state (when the user is not talking). In voice expansion, the transmit gain is adjusted infinitesimally between two limits with appropriate attack and release time constants so that there are no audible steps in the transmitted background noise level. Voice switching and voice expansion systems are developed primarily to suppress background noise, but with well-optimized voice expansion circuits in a quiet environment, up to 12 dB increase in echo path loss can be achieved with no audible artifact. However, voice expansion alone is insufficient for boomless headsets and the effectiveness of voice expansion is further diminished in noisy environments.
Another example of a conventional solution that attempts to resolve the problem of diminishing acoustic echo cancellation at higher frequencies is electronic echo cancellation with digital signal processing. In particular, an echo canceller adaptively predicts the echo signal and removes the predicted echo signal from the transmit path. However, such a method adds even more delay, is expensive to implement, consumes power, and can generate audible artifacts.
Thus, as the echo levels resulting from acoustic coupling in boomless and short-boom headsets can be high, it is desirable to provide electronic echo reduction for a communications headset used in digital networks with packet delay to ensure acceptable echo performance.